Similar estimates in the past have focused on deforestation, but the new study looks at the disappearance of a broader range of wild landscapes. “This is the first time that anyone has put a number on the loss,” says Tim Newbold, a conservation biologist at University College London whose own work has shown that wilderness areas contain the world’s most undisturbed biodiversity. Such unspoiled regions, scientists argue, are also critical for allowing the planet to cope with climate change. As a result, this new work is “a wake-up call,” says Robin Chazdon, an ecologist at the International Institute for Sustainability in Rio de Janeiro, Brazil, who was not involved with the study.

Watson, a conservation biogeographer at the Wildlife Conservation Society based at the University of Queensland, St. Lucia, in Brisbane, Australia, and his colleagues earlier determined the extent of the “human footprint” on Earth by incorporating maps and data on crop lands, pastures, night lighting, railways, roadways, navigable waterways, population densities, and “built” environments, which included urban areas and other settlements. For most of these threats to wilderness, they had satellite and other data from the early 1990s and for the late 2000s. All but two of these pressures have increased in that time, Watson and his colleagues reported in a study last month in Nature Communications. (Roadways and waterways haven’t expanded noticeably.)

Wilderness is defined as pristine landscapes mostly free of human disturbances, including roads. So for the new work, Watson’s team adjusted the wilderness detected in satellite images to exclude places experiencing these human pressures to come up with the total area of wilderness. They also excluded Antarctica and similar “rock and ice” habitats, the oceans, and large lakes.

By 2009, about 23% of Earth’s land remained as wilderness—about 30.1 million square kilometers spread mostly across North America, North Asia, North Africa, and Australia, they conclude today in Current Biology. That’s 3.3 million square kilometers less than in 1993, an area about twice the size of Alaska—Watson says. South America has lost almost 30% of its wilderness in that time and Africa has lost 14%. The losses included the total devastation of several large swaths of forest and swamp in the Congo and in New Guinea.

One reason for the trend, Watson and others say, is that governments and conservation organizations often prioritize their protection efforts on habitat that is severely threatened or degraded. But left unguarded, remote land is vulnerable to homesteading farmers and claim-seeking miners.

Moreover, by storing carbon and buffering local climate, wilderness areas, particularly forests, can be “by far and away the most effective way to deal with climate change,” adds Russ Mittermeier, executive vice chairman of Conservation International in Arlington, Virginia, who was not involved with the study. Harvard University biologist Edward O. Wilson is now arguing for the preservation of half Earth’s land (even if it’s not all wilderness) and oceans and Mittermeier and others think that protecting that fraction of forests can be “50% of the climate change solution.”

For all these reasons, Chazdon says, “protecting these areas will require a much stronger effort than has been made to date."